1. Field of Invention
The present invention relates to a motor drive control circuit, a semiconductor device, an electronic timepiece, and an electronic timepiece with a power generator.
2. Description of Related Art
Stepping motors and other types of motors are widely used as actuators in various different types of devices. In electronic timepieces, for example, a motor is used as an actuator to drive the hands.
Such motors have an operating voltage range, and the motor drive voltage is preferably kept constant within this operating voltage range, that is, the motor drive voltage is preferably a constant voltage, in order to drive the motor stably.
Primary batteries, such as silver oxide batteries that are conventionally used as the motor drive power source, provide a constant output voltage and can therefore easily supply a constant voltage as the motor drive voltage.
In order to make replacing the battery unnecessary, more recent electronic timepieces commonly have a built-in power generator such as a self-winding generator driven by a rotary pendulum or a solar generator, and a secondary battery that is charged by the current produced by the generator, and use the secondary battery as the primary power source of the motor.
However, because charging causes the voltage of the secondary battery to rise, the battery voltage may exceed the operating voltage of the motor.
To solve this problem, Japanese Unexamined Patent Appl. Pub. JP-A-H10-174494 teaches a motor drive control method for holding the motor drive voltage within the operating voltage range of the motor even when the secondary battery voltage rises by providing a voltage step-down circuit having three step-down capacitors and one smoothing capacitor in the power supply unit that controls the motor drive voltage, and supplying a voltage that is stepped down from the secondary battery voltage to the motor drive circuit.
Japanese Unexamined Patent Appl. Pub. JP-A-H07-306274 also teaches a drive control method that stabilizes motor drive by using an overcharge prevention mechanism (limiter) to suppress the voltage rise in the secondary battery caused by charging so that the motor drive voltage does not exceed a set constant voltage level.
There are two problems with the motor drive control method taught in Japanese Unexamined Patent Appl. Pub. JP-A-H10-174494.
First, because the step-down circuit reduces the secondary battery voltage by a specific multiple, the step-down voltage of the power supply unit also rises when the secondary battery voltage rises due to charging, and the motor drive voltage thus varies and does not remain constant.
More specifically, a large number of finely incremented step-down levels are required in order to maintain a constant step-down voltage as the secondary battery voltage rises analogically. However, setting numerous step-down levels requires numerous capacitors, which complicates the hardware circuit design.
Furthermore, if the step-down voltage is controlled using only two step-down ratios of 2/3 and 1/3 as taught in Japanese Unexamined Patent Appl. Pub. JP-A-H10-174494, the step-down voltage cannot be held constant as the secondary battery voltage rises on an analog curve.
Second, even if the step-down ratio from the state when the power supply unit supplies the battery voltage to the motor drive circuit is set to 2/3 based on the motor drive conditions so that the 2/3 step-down voltage is supplied to the motor drive circuit, it may not be possible to instantaneously step down the motor drive voltage because of the time constant of the smoothing capacitor in the power supply unit.
As a result, a delay that is at least as long as the time required for the motor drive voltage supplied form the power supply unit to go to the step-down voltage must be set in order to drive the motor stably. This means that the motor cannot be driven during this set delay period even when it is desirable to drive the motor immediately, and motor drive is thus delayed.
A problem with the method taught in Japanese Unexamined Patent Appl. Pub. JP-A-H07-306274 is that when the operating voltage range of the motor is relatively low, the limiter must operate in a low secondary battery voltage range and the duration time of the secondary battery is thus shortened. More specifically, because charging causes the voltage of the secondary battery to rise, the stored charge increases as the operating voltage of the limiter rises, and the duration time of the battery becomes longer. In other words, the voltage charge decreases and the duration time becomes shorter as the voltage at which the limiter operates decreases.
Therefore, if the limiter operates at a low voltage because the operating voltage range of the motor is low, the duration time is also shortened by a corresponding amount.
In order to balance stabilizing motor drive with increasing the duration time of the secondary battery, the operating voltage range of the motor must be increased so that the motor can be driven even when the secondary battery voltage is high.
However, increasing the operating voltage range of the motor normally imposes certain limits on motor performance. It is particularly difficult to achieve a wide operating voltage range in motors that operate at high speed in both forward and reverse rotation, and the motors that can be used for such applications are therefore limited.
Methods of preventing overcharging the secondary battery only while the motor is driven can also be used to stabilize motor drive, but this only stabilizes the motor drive voltage at the boosted secondary battery voltage, and ultimately requires increasing the operating voltage range of the motor. Furthermore, because the secondary battery cannot be charged while the motor is operating, charging efficiency drops.